Office chairs are known comprising a spoke-type base with feet or wheels and a column mounted on said base and provided with a spring, generally in the form of a gas piston, to adjust the height of the sitting plane from the floor.
A known type of such a base is obtained in one injection moulding step from thermoplastic materials, and presents a substantially U-shaped open cross-section to enable the male die punch to be extracted.
The current regulations regarding safety and reliability tests for chairs require that the base be subjected to a series of verification tests (compressions) to verify their structural strength and the absence of permanent deformations which could prejudice their integrity.
The load conditions to which a single base spoke is subjected can be schematically represented by likening the spoke to a beam fixed at one end to the central core and stressed by a vertical force acting upwards from below and applied at the point to which the wheel or foot is connected. This beam is therefore subjected to straight flexure with its lower fibres subjected to tension and its upper fibres to compression, and presents its maximum bending moment in correspondence with its fixed end, i.e. where the spoke joins the central core.
The base spokes are also subjected to twisting due to the misalignment between the wheel and the pin connecting the wheel to the base.
In those bases constructed in accordance with the known art the spokes are formed with an inverted U profile, i.e. with the material-lacking region lying precisely where the fibres are subjected to high tension forces.
Consequently to resist these stresses, the spokes are reinforced by increasing their thickness, adding reinforcement elements and inserting structural metal parts, or by using materials with better mechanical characteristics.
However all these additions result in considerable increases in material and manufacturing costs.
An object of the invention is to eliminate these drawbacks by providing a chair base presenting high resistance to the stresses concerned.
Another object of the invention is to provide a base which enables the price/performance trade-off to be shifted to a level not attainable by current bases present on the market.
These and other objects which will be apparent from the ensuing description are attained according to the invention by a method for forming office chair bases as described in claim 1.